Solvent hydrogen bonding and structural effects on the reaction of 2-halo-5-nitropyridines with parasubstituted anilines in dimethylsulfoxide/ acetonitrile mixtures

Article abstract of DOI:10.1002/kin.20558

Substitution reactions of some parasubstituted anilines with 2-chloro-5-nitropyridine and 2-bromo-5-nitropyridine were carried out conductometrically in dimethylsulfoxide/acetonitrile mixtures. The correlation of second order rate constants with Hammett's substituent constants yields a fairly linear straight line with a negative slope. The correlation of rate data with Kamlet-Taft's solvatochromic parameters is excellent (100R²= 97%) in both the substrates. The solvation model proposed is well supported by the solvatochromism exhibited by aniline in the solvent mixture under investigation. The molar extinction coefficient (ε_max) of aniline varies appreciably up to ～25% with the change in composition of the mixture. The multivariate correlation analysis of ε_max (with α, β, π) suggests that the solvation around NH₂ moiety of aniline through hydrogen bond donor (HBD) property is found to be dominant in the solvation process and consequently in altering the rate. The observation is that the dominance of HBD property in solvation is further confirmed by the cyclic voltammetric oxidation of aniline in the solvent mixture.

Full text of DOI:10.1002/kin.20558

Solvent Hydrogen Bonding
and Structural Effects on
the Reaction of 2-Halo-5-
nitropyridines with
Parasubstituted Anilines in
Dimethylsulfoxide/
Acetonitrile Mixtures
N. KALAIMANI, S. RATHINAM, D. S. BHUVANESHWARI
Department of Chemistry, Thiagarajar College, Madurai 625 009, India
Received 28 June 2010; revised 14 January 2011; accepted 24 January 2011
DOI 10.1002/kin.20558
Published online 10 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: Substitution reactions of some parasubstituted anilines with 2-chloro-5-
nitropyridine and 2-bromo-5-nitropyridine were carried out conductometrically in dimethyl-
sulfoxide/acetonitrile mixtures. The correlation of second order rate constants with Hammett’s
substituent constants yields a fairly linear straight line with a negative slope. The correlation of
rate data with Kamlet–Taft’s solvatochromic parameters is excellent (100R2 = 97%) in both the
substrates. The solvation model proposed is well supported by the solvatochromism exhibited
by aniline in the solvent mixture under investigation. The molar extinction coefficient (εmax
)
of aniline varies appreciably up to ∼25% with the change in composition of the mixture. The
multivariate correlation analysis of εmax (with α, β, π*) suggests that the solvation around NH2
moiety of aniline through hydrogen bond donor (HBD) property is found to be dominant in the
solvation process and consequently in altering the rate. The observation is that the dominance
of HBD property in solvation is further confirmed by the cyclic voltammetric oxidation of aniline
in the solvent mixture. ꢀ 2011 Wiley Periodicals, Inc. Int J Chem Kinet 43: 409–417, 2011
C
INTRODUCTION
of preferential solvation [1]. The study of influence
of solvent on the reactions of anilines in nonaqueous
and aquo-organic solvent mixtures has revealed the
important role of nonspecific and specific solvent ef-
fects on reactivity [2–7]. It has been established that
the technique of correlation analysis may well be used
to separate and quantify such solvent–solvent–solute
The study of solute–solvent interactions in binary mix-
tures is more complex than in pure solvents as a result
Correspondence to: D. S. Bhuvaneshwari: e-mail: bukanch@
yahoo.co.in.
c
ꢀ
2011 Wiley Periodicals, Inc.

410
KALAIMANI, RATHINAM, AND BHUVANESHWARI
interactions on reactivity. Further, one of the impor-
tant tools in deciding the mechanism of reactions is
the study of substituent effects through the Hammett
equation and its modified forms [8].
Spectral measurements: The solution FT-IR exper-
iment was done with a horizontal attenuated total
reflectance ZnSe flat prism plate, in a JASCO FT-
IR 460 Plus spectrometer. The electronic absorption
spectra are recorded on a JASCO double beam spec-
trophotometer using 1 cm matched quartz cells. The
electrochemical experiments were performed at 25◦C
using a standard three-electrode, two compartment
configuration with a glassy carbon (GC–3 mm) work-
ing electrode, a spiral platinum counter electrode, and
a Ag(AgCl (KCl sat.) reference electrode. The car-
bon electrodes were polished between experiments
with alumina (0.5 μm) paste. The cyclic voltammet-
ric experiments were carried out with a computer-
controlled electrochemical system (CHI643B Electro-
Examination of the literature revealed that the ef-
fects of structure on SNAr2 reactions have largely
been reported [9–23]. However, only very few attempts
have been made to study on the effect of solvent on
such reactions in a more systematic manner [24–30].
Hence, in this article, the reaction of few parasub-
sutituted anilines with 2-bromo-5-nitropyridine and
with 2-chloro-5-nitropyridine in dimethylsulfoxide
(DMSO)/acetonitrile (AcN) (both polar, aprotic, Hy-
drogen Bond Acceptor, PAHBA solvents) mixtures
of varying compositions has been reported. To study
the effect of solvent and structure on the reactiv-
ity 2-bromo-5-nitropyridine (BNP) and 2-chloro-5-
nitropyridine (CNP) are chosen as the substrate, in the
present investigation, as the kinetics of its reaction with
aniline in neat organic solvents is known [31]. Further,
the selection of DMSO-AcN solvent mixture for the
study is based on the fact that the hydrogen bonding
abilities of the mixture is well understood and among
the PAHBA solvents, AcN exhibits a lower HBA abil-
ity and also exhibits a potential ability to donate a
hydrogen atom toward the formation of a hydrogen
bond [32].
chemical Analyzer) at 50 mV s−1
.
Linear solvation energy relationships: The rate
data were correlated with Reichardt’s ET(30) polar-
ity parameter [33]. The most celebrated Kamlet–Taft
solvatochromic comparison method [34]. These sol-
vatochromic parameters for the solvent mixtures em-
ployed in the present study were obtained from the
literature [32].
Dataanalysis: Correlationanalyses were carriedout
using Microcal Origin (version 6) computer software.
The goodness of the fit was discussed using corre-
lation coefficient and standard deviation, SD [8]. The
percentage contribution (PX) of a parameter to the total
effect on reactivity was computed using the regression
coefficient of each parameter as reported earlier [35].
EXPERIMENTAL
Product analysis: The product analysis was carried
by refluxing a mixture of BNP/CNP (3.16 mmol) and
aniline (5.38 mmol) for an hour in DMSO (5 mL).
The mixture was cooled to room temperature and then
poured into ice-cold water with vigorous stirring; the
precipitate formed was filtered and recrystallized from
benzene-ether mixture (yield 83%). The product thus
obtained was subjected to GC-MS analysis. The re-
sults of the GC-MS analysis reveal that the reaction
product was 2-phenylamino-5-nitropyridine (m/z 215;
M+.) and its fragmentation with m/z ratio 123 and 92,
respectively.
Materials: All the chemicals used were of analyt-
ical grade (Aldrich or Merck, India). The solvents
dimethylsulfoxide and acetonitrile were of chromato-
graphic grade and used as received.
Kinetic studies: The reactions of 2-halo-5-
nitropyridines with substituted anilines in varying mole
fractions of acetonitrile in dimethylsulfoxide were fol-
lowed conductometrically at 30, 40 and 50( 0.1)◦C.
Pseudo-first-order conditions were used in all cases,
that is, [Aniline] > [BNP or CNP]. The reaction is so
slow that it is inconvenient to wait for its completion.
Therefore, the Guggenheim method [29] was used to
evaluate the rate constants by carrying out the kinetic
runs for up to 3 h. The second-order rate constants,
kA, were obtained from the observed rate constants
as reported earlier [28]. Regression coefficients of all
the reaction rate constants were around 0.99. All rate
determinations were carried out at least in duplicate
RESULTS AND DISCUSSION
The nucleophilic substitution reaction of parent ani-
line and a few para- (Et, Me, OMe, F, Cl and NO2)
substituted anilines with BNP and CNP was studied
conductometrically at 303, 313, and 323 K in the pres-
ence of varying excess [aniline] over the substrate to
ensure pseudo-first-order kinetics. The selection of the
mole fractions for the reaction was based on the fact
and the rate constants are accurate to within
3%.
The product analysis was carried out by employing the
GC-MS technique.
International Journal of Chemical Kinetics DOI 10.1002/kin

STRUCTURAL EFFECTS ON THE REACTION OF 2-HALO-5-NITROPYRIDINES
411
Table I Effect of Temperature on the Rate of Reaction of Anilines with 2-Bromo-5-nitropyridine in 0.2 Mole Fraction
of DMSO and Activation Parameters for the Reaction
Rate Constants 103 k2, s−1
Substituents
in Aniline
303 K
313 K
323 K
ꢀH#
ꢀS#
ꢀG# (303 K)
None
p-Et
p-Me
p-OMe
p-F
1.91
2.34
2.39
2.23
2.12
1.73
1.56
2.11
2.69
2.52
2.32
2.26
1.91
1.83
2.57
4.55
2.56
2.59
3.65
2.11
4.04
09.6
24.7
01.5
03.5
19.7
05.6
36.5
−271
−219
−295
−289
−237
−285
−184
91.7
91.0
91.1
91.3
91.6
91.9
92.5
p-Cl
p-NO2
ꢀH# in kJ mol−1; ꢀS# in J K−1 mol−1; ꢀG# kJ mol−1
[aniline] = 0.1 M [BNP] = 0.001 M.
.
that the solvatochromic parameters employed in the
present study for correlation analysis are available in
the literature [32]. Based on the kinetic results, product
analysis, and the conclusions of previous work [31], the
following reaction scheme has been proposed for the
reaction of anilines with the substrate, which involves
an intermediate (I). See Scheme 1. The formation of
such a type of intermediate was well established in
the SNAr2 reactions of aromatic amines with various
substrates [9–13].
The solution FT-IR spectrum of the reaction mix-
ture at different time intervals was recorded and the
doublet around 2930–3000 cm−1 corresponds to asym-
metric and symmetric stretching vibrations of the two
N-H bonds of the aromatic primary amine [39]. It is
observed that with lapse of time the intensity of the
peak decreases, which confirms the participation of
the amine group in the reaction.
Thermodynamic Parameters and the
Isokinetic Relationship
The influence of [aniline] on the rate of the reac-
tion was studied with an aim to determine the rate-
limiting step of the reaction. As a representative case,
The activation parameters for the reaction of all the
substituted anilines with BNP at 0.2 mole fraction of
DMSO and with CNP at 0.6 mole fraction of DMSO
were calculated from kA at 303, 313, and 323 K us-
ing the van’t Hoff plot by the method of least squares
and are collected in Tables I and II. The reaction is
neither isoenthalpic nor isoentropic but complies with
the compensation law also known as isokinetic rela-
tionship. The isokinetic temperature is the temperature
the second-order rate constants (103 kA, −1) for the
s
reaction of aniline with BNP (0.001 M) at 0.2 mole
fraction of DMSO in AcN at 303 K were observed to
be 1.91, 1.89, 1.93, 1.90, and 1.89 at [aniline] = 0.1,
0.15, 0.2, 0.25, and 0.3 M, respectively. The results
indicated that there is no increase in the rate of the
reaction with an increase in [aniline] and hence the
reaction is not catalyzed by a base [36–38].
Table II Effect of Temperature on the Rate of Reaction of Anilines with 2-Chloro-5-nitropyridine in 0.6 Mole Fraction
of DMSO and Activation Parameters for the Reaction
Rate Constants 103 k2, s−1
Substituents
in Aniline
303 K
313 K
323 K
ꢀH#
ꢀS#
ꢀG# (303 K)
None
p-Et
p-Me
p-OMe
p-F
1.59
1.98
1.68
1.67
1.33
0.45
0.28
1.90
2.41
2.13
1.97
1.48
1.02
1.64
2.06
3.07
3.40
2.86
1.96
1.26
2.66
08.3
15.6
26.5
19.1
13.3
40.4
91.2
−276
−251
−216
−239
−262
−180
−16
92.1
91.6
92.0
91.8
92.7
95.2
91.5
p-Cl
p-NO2
ꢀH# in kJ mol−1; ꢀS# in J K−1 mol−1; ꢀG# kJ mol−1
[aniline] = 0.1 M [CNP] = 0.001 M.
.
International Journal of Chemical Kinetics DOI 10.1002/kin

412
KALAIMANI, RATHINAM, AND BHUVANESHWARI
Table III Rate Constants (103k2, s−1) and Activation Parameters for the Reaction of Aniline with
2-Bromo-5-nitropyridine in Various Mole Fractions of DMSO in AcN
Rate Constants 103k2, s−1
Mole Fraction of DMSO
303 K
313 K
323 K
ꢀH#
ꢀS#
ꢀG# (303 K)
0
1.63
1.73
1.91
2.21
2.25
2.30
2.30
2.44
2.50
2.55
2.70
2.18
2.01
2.11
3.35
2.44
2.45
2.51
2.66
2.77
2.90
3.10
2.85
2.51
2.57
4.03
3.16
2.82
2.95
3.34
3.23
3.70
3.80
20.6
12.7
09.6
22.4
11.3
05.8
07.6
10.3
08.0
12.7
11.5
−241
−261
−271
−227
−264
−282
−276
−266
−274
−258
−261
91.6
91.8
91.7
91.2
91.3
91.2
91.3
90.9
91.0
91.0
90.8
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
ꢀH# in kJ mol−1; ꢀS# in J K−1 mol−1; ꢀG# kJ mol−1
.
at which all the compounds of the series react equally
fast. Also, at the isokinetic temperature the variation of
substituent has no influence on the free energy of ac-
tivation. In an isoentropic reaction, the isokinetic tem-
perature lies at infinite and only enthalpy of activation
determines the reactivity. The isokinetic temperature
is zero for an isoenthalpic series and the reactivity is
determined by the entropy of activation [40]. The oper-
ation of isokinetic relationship is tested by plotting the
logarithms of rate constants, kA, at two temperatures
(T2 > T1) against each other according to Eq. (1) as
suggested by Exner [41].
In the present study, linear plots imply the va-
lidity of isokinetic relationship (Reaction with BNP,
r = 0.97, SD = 0.016, isokinetic temperature = 380
14 K and Reaction with CNP, r = 0.91, SD = 0.06,
isokinetic temperature = 231 16 K). The existence
of isokinetic relationship reveals the operation com-
pensation effect in the present system [42]. Negative
entropy of activation indicates a greater degree of or-
dering in the transition state than in the initial state,
due to an increase in solvation during the activation
process.
Likewise, the activation parameters were also cal-
culated for aniline in different mole fractions of AcN
(Tables III and IV). The results in Tables III and IV
reveal that increase in mole fraction of DMSO in the
log k (at T2) = a + b log k (at T1)
(1)
Table IV Rate Constants (103 k2, s−1) and Activation Parameters for the Reaction of Aniline with
2-Chloro-5-Nitropyridine in Various Mole Fractions of DMSO in AcN
Rate Constants 103 k2, s−1
Mole Fraction of DMSO
303 K
313 K
323 K
ꢀH#
ꢀS#
11
ꢀG# (303 K)
0
0.42
0.70
0.85
1.32
1.46
1.87
1.59
2.09
2.40
2.95
3.23
1.25
1.61
3.07
2.62
2.52
3.33
1.90
3.17
3.20
3.55
3.71
4.98
5.05
5.33
3.02
5.12
9.87
2.06
6.87
4.31
4.57
5.20
99.1
78.7
73.7
32.0
49.3
66.1
08.3
46.5
21.5
15.4
17.0
95.7
94.4
93.5
92.4
92.5
92.0
92.1
91.6
91.1
90.6
90.4
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
−51
−65
−199
−142
−85
−276
−148
−229
−248
−242
ꢀH# in kJ mol−1; ꢀS# in J K−1 mol−1; ꢀG# kJ mol−1
.
International Journal of Chemical Kinetics DOI 10.1002/kin

STRUCTURAL EFFECTS ON THE REACTION OF 2-HALO-5-NITROPYRIDINES
413
–2.60
–2.65
–2.70
–2.75
–2.80
–2.7
r = 0.93
sd = 0.14
r = 0.91
sd = 0.03
= -0.18
ρ
= -0.87
–3.0
–3.3
–3.6
-0.3
0.0
0.3
0.6
0.9
-0.3
0.0
0.3
0.6
0.9
σ
Figure 1 Hammett plot of log kA versus σ for the reaction
of anilines with BNP in 0.2 mole fraction of DMSO at 303 K.
Figure 2 Hammett plot of log kA versus σ for the reaction
of aniline with CNP in 0.6 mole fraction of DMSO at 303 K.
O
N
R
medium increased the rate of the reaction. The exis-
tence of a linear relationship (r = 0.96, SD = 0.019,
isokinetic temperature = 685 16 K between log kA
at 323 K and log kA at 313 K in BNP and r = 0.92,
SD = 0.06, isokinetic temperature = 324 19 K be-
tween log kA at 313 K and log kA at 303 K in CNP)
indicated the operation compensation effect in the re-
action under investigation [42].
N+
X
H2N
+
-O
Aniline
2-halo-5-nitropyridine
k1
k-1
O
N
X
H
R
N+
Structure-Reactivity Correlation
N
-O
The effect of substituents on the rate was studied with
seven parasubstituted anilines. The plot of log kA ver-
sus Hammett’s substituent constants is satisfactory in
the solvent composition studied (Figs. 1 and 2). The
negative slope indicated the involvement of a positively
charged intermediate during the reaction [8]. This is,
in the present study, obviously the nitrogen of the NH2
group. This facilitates the bond-breaking process in the
intermediate [32].
H
(I)
-HX
k2
k3[B]
O
N+
R
-O
N
Solvent-Reactivity Correlation
N
H
The title reaction has been studied in 11 different mole
fractions of DMSO in AcN. The second order rate
constants increased with an increase in mole fraction of
DMSO in the mixture. The correlation of rate data with
Reichard’s [33] polarity parameters ET(30) is just sat-
isfactory with an explained variance of less than 93%.
Such a correlation may be due to the fact that bulk
solvent properties like ET(30) will poorly describe the
microenvironment around the reacting species, which
governs the stabilityof thetransitionstate and hence the
5-nitro-N-phenylpyridin-2-amine
Scheme 1
rate of the reaction. Parallel observations has already
been made in the Cr(VI) oxidation [5,6,43–45], substi-
tution [46], and electrochemical oxidation [47,48] of
anilines in mixed solvents.
Therefore, in order to obtain a deeper insight into the
various solute–solvent interactions, which influence
International Journal of Chemical Kinetics DOI 10.1002/kin

414
KALAIMANI, RATHINAM, AND BHUVANESHWARI
the solvatochromic parameters failed to correlate either
with electrical parameters or size of the substitutents.
This may be due the fact that the solvation depends
on various factors like the cavity that the dissolved
molecule produces in the solvent, the orientation of
the solvent molecules and the unspecific, and specific
intermolecular forces [33]. Hence, the percentage con-
tribution of a particular solvatochromic parameter, for
given aniline, may be due to the combination of one
or more factors mentioned the average value was em-
ployed for further discussions. Also, a majority of the
data is in line with the above mentioned average.
The statistical results of the correlation of the rate
of the reactions of aniline with 2-halo-5-nitropyridines
in DMSO/AcN mixtures and the resultant percentage
contributions of the solvatochromic parameters ((Eqs.
(2) and (3)) indicated that the reactivity is influenced
by both nonspecific and specific solute–solvent interac-
tions. As AcN possesses hydrogen bond donor (HBD)
property, it can solvate the NH2 moiety of aniline
through specific solute–solvent interactions. Increase
in the mole fraction of DMSO in the mixture should
have progressively removed the solvation around the
NH2 moiety, allowing the reaction partner to approach
the aniline relatively easier and consequently increased
the rate of the reaction as observed. The observed
variation of the rate constants with an increase in the
mole fraction of DMSO (Tables III and IV) indicates
that the increase in the rate constant, particularly be-
yond 0.4 mole fraction of DMSO, is not smooth. This
observation may be due to the occurrence of strong
solvent–solvent interactions, mainly in the DMSO-rich
region, rather than solute–solvent interactions. Hence,
in the present study in addition to solute–solvent in-
teraction, solvent–solvent interaction also played a vi-
tal role in governing the reactivity of anilines [32].
An attempt was also made to employ UV–visible
and cyclic voltammetric techniques to support the
presumption.
I (S1)m + m S2
I (S1)m + m/2 S2
I (S2)m + mS1
I (S12)m + m/2 S1
Scheme 2
reactivity, we have tried to adopt the solvatochromic
comparison method developed by Kamlet and Taft
[34]. The solvatochromic parameters employed in the
present study were computed and reported [32] based
on various solvent–solvent interactions, which can be
explained by the following general model and is based
on a two solvent exchange processes (Scheme 2) [1].
I (S1)m + mS2
I (S1)m + m/2S2
ꢀ
I (S2)m + mS1
I (S12)m + m / 2 S1
ꢀ
where S1 and S2 indicate the two pure solvents to be
mixed and S12 represents a solvent formed by the in-
teraction of solvents 1 and 2. This new solvent can
have properties quite different from those of solvents 1
and 2 for synergetic mixtures. The term m is the num-
ber of solvent molecules solvating the solvatochromic
indicator I [1].
The kinetic data were correlated with the Kamlet–
Taft’s solvatochromic parameters α, β, and π∗. The
rates of the reaction of aniline with both BNP and CNP
studied showed an excellent correlation with the sol-
vent with an explained variance of 97%. The statistical
results and weighted percentage contributions of the
solvatochromic parameters are shown in the following
equations. Such an excellent correlation indicates the
existence of specific solute–solvent interactions in the
reactions.
With BNP:
log kA = −2.73( 0.27) − 0.076( 0.33)π∗
− 0.46( 0.22)α + 0.29( 0.10)β (N = 11,
R2 = 0.97, SD = 0.016, Pα = 56%,
∗
The UV–visible absorption spectra of the reactants
in the solvent mixture and the solvatochromism ex-
hibited by them adequately support the conclusions
derived above. The UV–visible absorption spectra of
aniline in different mole fractions of DMSO has been
recorded and the wavelengths (λmax) and molar ex-
tinction coefficients (εmax) of maximum absorption of
aniline in these solvent mixtures are summarized in
Table V. It is evident from the results that the peak po-
sition of UV absorption (λmax) shows little dependence
on the solvent (<1%). In contrast, the absorption in-
tensity (εmax) exhibits a significant increase (>25%)
from 2740 M−1 cm−1 in AcN to 3705 M−1 cm−1 in
DMSO. Thus, εmax is likely to be affected by hydrogen
bonding [49].
Pβ = 35%, Pπ = 09%)
(2)
With CNP:
log kA = −4.81( 1.05) + 1.76( 1.30)π∗
− 0.43( 0.84)α + 0.66( 0.37)β (N = 11,
R2 = 0.97, SD = 0.06, Pα = 15%,
∗
Pβ = 23%, Pπ = 62%)
(3)
The percentage contribution of the three solva-
tochromic parameters depends on the nature of the sub-
stituents. However, the percentage contribution data of
International Journal of Chemical Kinetics DOI 10.1002/kin

STRUCTURAL EFFECTS ON THE REACTION OF 2-HALO-5-NITROPYRIDINES
415
Table V Absorption Maxima for Aniline in Different
Mole Fractions of DMSO in AcN
9.0x10-4
6.0x10-4
Mole Fraction
of DMSO
Aniline
(nm)
ε
M−1
max
cm−1
λ
max
0
290.9
2740
2775
2885
2935
2960
3115
3325
3365
3375
3455
3705
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
292.3
292.4
292.8
292.9
293.5
293.8
294.5
294.7
294.5
294.9
3.0x10-4
0.0
0.3
0.6
0.9
1.2
1.5
E (V)
Figure 3 Cyclic voltammogram for the oxidation of aniline
(1 × 10−3 M) in different mole fractions of DMSO at a scan
rate of 50 mV s−1. (The direction of the arrow indicates the
shift in oxidation potential with increase in mole fraction of
DMSO.)
This observation can qualitatively be explained as:
since DMSO is a nonhydrogen-bonding solvent while
AcN exhibits a potential ability to donate a hydrogen
atom toward the formation of an H-bond, the latter can
interact with aniline through HBD property but not the
former one. This point of observation has also been
explained quantitatively through correlation analysis.
The intermolecular solute–solvent interactions through
H-bonding were examined by correlating εmax of ani-
line against the Kamlet–Taft’s solvatochromic param-
eters α, β and π∗ ((Eq. (4)), as this equation comprises
both the HBD and HBA terms in it. The result of the
correlation is represented as
fraction of DMSO might be due to the desolvation
of the NH2 moiety to a relatively greater extent. Such
a solvation of aniline by AcN through HBD property
around the reaction center would make the approach
of the reaction partner comparatively difficult and con-
sequently retard the rate of the overall reaction.
The above-mentioned fact was further supported by
cyclic voltammetric studies of aniline in the given sol-
vent mixtures. The cyclic voltammogram of aniline
with varying mole fraction of DMSO is depicted in
Fig. 3. The anodic peak at ∼1.3 V corresponds to the
electrooxidation of aniline. This oxidation potential
shifts toward the less positive side with an increase in
the mole fraction of DMSO in the mixture suggesting
that the removal of electron from NH2 moiety (oxida-
tion) becomes increasingly easier. A plot of oxidation
potentials, Epa, versus mole fraction of DMSO is linear
(r = 0.96, SD = 0.009) with negative slope (Fig. 4).
The decrease in the oxidation potential of aniline with
a decrease in non-HBD property of the medium was
observed earlier by us in the study of electrooxida-
tion of anilines in water/2-methylpropan-2-ol [47] and
water/acetic acid [48] mixtures.
To conclude, from the foregoing results and discus-
sion, with an increase in the mole fraction of acetoni-
trile (a solvent with relatively higher HBD property)
in the mixture the pair of electrons on the NH2 moiety,
through H-bonding, is solvated to a greater extent and
makes its participation in the formation of the inter-
mediate (I) difficult and consequently decelerate the
log εmax = 3.15( 0.17) + 0.44( 0.21)π∗
− 0.14( 0.04)α − 0.05( 0.01)β (N = 11,
R2 = 0.96, SD = 0.01, Pα = 22%,
∗
Pβ = 8%, Pπ = 70%)
(4)
It can be seen from the results that the absorption in-
tensity is influenced by both specific and nonspecific
solute–solvent interactions as indicated by the percent-
age contributions of α, β, and π∗ parameters. Among
specific interactions, the contribution of α term is found
to be dominant and the negative sign of coefficient of
this term suggests that the absorption intensity would
decrease with increase in HBD property of the medium.
The contribution of HBA property (Pβ) toward the sol-
vation of aniline by the solvent mixture is less signifi-
cant. Thus increase in the mole fraction of DMSO in the
mixture progressively decreases the solvation around
the NH2 moiety of aniline molecule and consequently
increases the absorption intensity. Hence, the observed
increase in rate of the reaction between aniline and
2-halo-5-nitropyridines with increase in the mole
International Journal of Chemical Kinetics DOI 10.1002/kin

416
KALAIMANI, RATHINAM, AND BHUVANESHWARI
1.35
1.32
1.29
1.26
1.23
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